It is conventional to mount a rotary switch on the transmission housing externally thereof to receive mechanical gear selection inputs from the operator of the vehicle through various linkages and output gear selections to a decoder module and the transmission electronic control unit via electronic signals. The switch includes a quadrant with a selected number of electrical contact segments disposed thereon with each segment providing an output to the decoder module. The manual valve controlling hydraulic operation of the transmission is mechanically coupled to a plate having indexing detents, called a detent lever, mounted on a shaft and pivotably movable therewith. The shaft extends through the transmission housing wall and a switch bar is fixedly attached to the shaft externally of the transmission housing so that when a vehicle operator selects a gear, the switch bar within the rotary switch moves across the quadrant to a predetermined position to engage one or more of the electrical contact segments. In certain systems the electronic control monitors the gear position along with other inputs relative to such things as throttle position, output shaft speed, engine speed, engine load and so on.
Although this switch system is widely used it suffers from certain limitations and disadvantages. Securing the switch housing to the transmission housing in correct angular orientation, either during initial installation or in-service, requires either special tools to align a reference feature of the shaft following component, i.e., the switch bar, to the rotary switch housing or it entails the monitoring of electrical continuity of switch contacts while rotating the housing relative to the shaft. Additionally, locating the switch outside the transmission housing and removed from the manual valve results in an undesirable tolerance build-up involving all the linking elements. Further, the switch is located in a splash zone, particularly when used with four wheel drives, necessitating the provision of sealing means to prevent entry of water. The extra cost for such sealing means including venting tubes and the like adds significantly to the cost of the system.
In copending application Ser. No. 805,215, incorporated herein by means of this reference, a switch system is disclosed and claimed comprising pressure switches mounted on a bracket within the transmission housing and coupled to selected hydraulic passages and having electrical circuit traces on the bracket. A movable contact is operatively connected to the manual valve and is movable therewith so that the position of the movable contact tracks the manual valve position and moves into and out of engagement with stationary contact segments disposed on the bracket. The pressure switch outputs are incorporated into a resistor network with the manual valve position sensor. The system provides a voltage output to the powertrain electronic controller which interprets the output as a particular transmission event, for example, a gear selection.
While this latter system is effective to provide an indication of the gear selected it cannot be used over the full stroke in certain transmission assemblies in which the valve moves inside of the housing. Further, there is a need to provide relatively high and low current switching dependent upon particular gear selections, for example to permit starting only in specific gear selections, to energize reverse lights when in reverse gear, to energize park accessories when in park, etc.
External switches mounted outside the transmission housing or on the operator gear selector have been used for this purpose; however, they suffer from the same problems described above in relation to the position sensors. That is, switches mounted externally of the transmission housing are located in a hostile environment subject to water splash and the like resulting in significant high warranty costs. Further, such switches have inherent problems relating to difficulty involved in installation and are susceptible to changes in alignment relative to the manual valve, particularly switches located in the passenger compartment.
It has been proposed to locate a switching system within the transmission housing using discrete magnets attached to the detent lever along with fixedly positioned magnetic sensors to control energization of the several circuits dependent upon the position of the manual valve; however, temperature changes affect their operation and changes occur to the magnets themselves over time. Further, electromagnetic compatibility of the system presents a problem. The transmission fluid tends to pick up ferromagnetic particles generated by rotating components such as gears which are attracted to the magnets thereby adversely affecting switching sensitivity.
It has also been proposed to place conductive traces on the detent lever and position wipers to engage the traces as the detent lever pivots; however, this has not been satisfactory for several reasons including the difficulty of assuring adequate contact force as well as environmental concerns.
It is therefore an object of the invention to provide an electric switching assembly dependent upon gear selection which is free of the limitations of the prior art mentioned above. Another object is the provision of a switch system which can be located within the transmission housing of a vehicle which is simple, reliable and has improved longevity. Yet another object is the provision of such a system which is easily installed and which requires no post installation adjustment.
Other objects, advantages and details of the transmission sensor and switch system of this invention appear in the following detailed description of preferred embodiments of the invention.